Two distinct processes involving DNA double-strand breaks (DSB) have been identified in mammalian cells: the repair of DNA damage induced by ionizing radiation and V(D)J recombination during T- and B-cell development. So far, all mammalian cell mutants defective in DNA DSB repair share the common phenotype of hypersensitivity to radiation, and impaired ability to undergo V(D)J recombination (1-6). Cell fusion studies using DSB repair mutants of human-rodent somatic hybrids have defined four complementation groups: IR4, IR5, IR6, and IR7. Genetic and biochemical analyses have revealed that cells of IR5 (e.g., xrs-6) and IR7 (e.g., scid) are defective in components of the DNA-dependent protein kinase (DNA-PK) (2, 7-9). DNA-PK is a serine/threonine kinase comprised of a large catalytic subunit (DNA-PKcs) and a DNA-targeting component termed Ku, which itself is a heterodimer of a 70-kDa (Ku70) and a 86-kDa (Ku80) polypeptide (10-12). Recently, DNA-PKcs has been shown to be the gene responsible for the murine scid (severe combined immunodeficiency) defect (13-15); and Ku80 has been identified to be XRCC5 (16-18), the X-ray-repair cross-complementing gene for IR5. Ku80 knockout mice were found to exhibit severe combined immunodeficiency, defective processing of V(D)J recombination intermediates, and growth retardation (19, 20).
Though Ku70 has been designated as XRCC6 (7, 8) and is an important component of the DNA-PK complex, the function of Ku70 in vivo is hitherto unknown. To define the role of Ku70 in DNA repair and V(D)J recombination, we targeted the Ku70 gene in mice. Ku70 homozygotes exhibit proportional dwarfism, a phenotype of Ku80−/−, but not of scid mice. Absence of Ku70 confers hypersensitivity to ionizing radiation and deficiency in DNA DSB repair, which are characteristics of both Ku80−/− and scid mice.
Surprisingly, in contrast to Ku80−/− and scid mice, in which both T- and B-lymphocyte development are arrested at early stage, lack of Ku70 is compatible with T cell receptor gene recombination and the development of mature CD4+CD8− and CD4−CD8+ T cells. Our data, for the first time, provide direct evidence supporting that Ku70 plays an essential role in DNA DSB repair, but is not required for TCR gene recombination. These results suggest that distinct but overlapping repair pathways may mediate DSB repair and V(D)J rejoining; furthermore, it suggests the presence of a Ku70-independent rescue pathway in TCR V(D)J recombination. The distinct phenotype of Ku70−/− mice should make them valuable tools for unraveling the mechanism(s) of DNA repair and recombination.
Ku is a complex of two proteins, Ku70 and Ku80, that functions as a heterodimer to bind DNA double-strand breaks (DSB) and activate DNA-dependent protein kinase (DNA-PK). The role of the Ku70 subunit in DNA DSB repair, hypersensitivity to ionizing radiation and V(D)J recombination was examined in mice that lack Ku70(Ku70−/−). Like Ku80−/− mice, Ku70−/− mice showed a profound deficiency in DNA DSB repair and were proportional dwarfs. Surprisingly, in contrast to Ku80 −/− mice, in which both T- and B-lymphocyte development were arrested at early stage, lack of Ku70 was compatible with T cell receptor gene recombination and the development of mature CD4+CD8− and CD4−CD8+ T cells. Our data shows, for the first time, that Ku70 plays an essential role in DNA DSB repair, but is not required for TCR V(D)J recombination. These results suggest that distinct but overlapping repair pathways may mediate DNA DSB repair and V(D)J recombination.